1. Field of the Invention
The invention generally relates to laser diode systems and, more specifically, to a laser diode driver circuit which provides an automatic power control (APC) for a laser diode for use in laser diode systems such as a photo pickup, an optoelectronic transceiver, etc.
2. Description of the Prior Art
As is well known in the art, the laser diode changes in the driving current-to-optical output power characteristic due to temperature variation or aging. For this reason, there have been proposed various techniques of feedback controlling the current flowing through a laser diode on the basis of an error or difference between a reference value and a detected value of the optical output power emitted by the laser diode so as to keep constant the extinction ratio, i.e., the ratio between the maximum output power at the time of light emitting operation of the laser diode and the minimum output power at the time of light extinguishing operation of the laser diode. Hereinafter, the level of the maximum output power, which corresponds to binary ones, is referred to as the peak level; and the level of the minimum output power, which corresponds to binary zeros, is referred to as the bottom level.
One of such techniques is disclosed, for example, in Japanese patent publication No. 2,856,247 (1998). FIG. 1 is a diagram showing the arrangement of a prior art laser diode (LD) driving circuit 0 of the APC method disclosed in the publication. In FIG. 1, the LD driving circuit comprises: an LD 1; a monitoring photodiode 2 so positioned and oriented as to be optically coupled with LD 1 to produce a photo-current Ipd; a current-to-voltage (C/V) converter 3 for converting the photo-current Ipd into a detected voltage Vo, which is directly proportional to the optical power transmitted by LD 1; a peak error detector 4 for providing a peak error signal indicative of the error or difference between a peak reference voltage Vpeak and each peak value Vop of the detected voltage Vo; a bottom error detector 5 for providing a bottom error signal indicative of the error or difference between a bottom reference voltage Vbottom and each bottom value Vob of the detected voltage Vo; a bias current driver 9 for supplying LD 1 with a bias current Ib of a level for an extinction operation of LD 1; and a modulation current driver 8 for supplying LD 1 with a modulation current Im which, together with the bias current Ib, constitutes a driving current at the time of a light emitting operation of LD 1.
Bias current driver 9 comprises a current source 91 having a control input terminal, which is supplied with the bottom error signal from the bottom error detector 5, and accordingly having a capability of providing a bias current Ib variable in response to the bottom error signal. Modulation current driver 8 comprises: a differential pair of transistors 81 and 82 having their gates supplied with a differential pair of modulation signals Data and {overscore (Data)} and having their drains connected with the cathode terminal of LD1 and the power supply Vcc, respectively; and a current source 83 having its take-in terminal connected with the source terminals of transistors 81 and 92, its output terminal coupled to the ground, and its control input terminal supplied with the peak error signal from the peak error detector 4, thereby to be capable of causing a current Ip variable in response to the peak error signal. Since modulation current driver 8 causes the current Ip to flow through LD1 only when the modulation signal Data is high or logical xe2x80x9c1xe2x80x9d, a modulation signal Data of a low level or logical xe2x80x9c0xe2x80x9d will cause a driving current Ib to flow through LD 1 thereby to cause LD 1 to be inactive, and a modulation signal Data of the high level or logical xe2x80x9c1xe2x80x9d will cause a driving current Ib+Ip to flow through LD 1 thereby to cause LD 1 to radiate an optical energy. It is noted that the differential modulation signals Data and {overscore (Data)} are a pair of binary signals which change in a complementary manner between a higher level and a lower but non-zero level as shown by the two upper graphs in FIG. 13.
A portion of the optical energy emitted from LD 1 is detected by photodiode 2 as a photo-current Ipd, which is converted by C/V converter 3 into a voltage Vo proportional to the emitted optical energy. The voltage Vo is supplied to the above-mentioned peak 4 and bottom 5 error detectors.
The peak error detector 4 comprises: a peak hold circuit 41 for detecting and holding the peak level Vop of the output voltage Vo from C/V converter 3; an operational amplifier 42; and a feed-back resistor 43 connected between the output terminal and the inverting input terminal of op-amp 42. The non-inverting input terminal of op-amp 42 is connected with the node between serially connected resistors 11 and 12 spanning the power supply potential Vcc and the ground thereby to be supplied with a reference voltage Vpeak. Op-amp 42 has its inverting input terminal further coupled with the peak hold circuit 41 output Vop and its output terminal further connected with the control input of the current source 83 within modulation current driver 8 so as to supply a peak error signal based on the difference between the peak level Vop and the reference voltage Vpeak to the current source 83 control input. The magnitude of the current Ip which modulation current driver 8 flows through LD 1 is controlled such that the peak level (or envelope) Vop of the C/V converter 3 output Vo matches the reference voltage Vpeak, i.e., the peak error signal from op-amp 42 becomes zero in the level.
The bottom error detector 5 comprises: a bottom hold circuit 51 for detecting and holding the bottom level Vob of the output voltage Vo from C/V converter 3; an operational amplifier 52; and a feed-back resistor 53 connected between the output terminal and the inverting input terminal of op-amp 52. The non-inverting input terminal of op-amp 52 is connected with the node between serially connected resistors 13 and 14 spanning the peak hold circuit 41 output Vop and the ground thereby to be supplied with a reference voltage Vbottom. Op-amp 52 has its inverting input terminal also coupled with the bottom hold circuit 51 output Vob and its output terminal also connected with the control input of the current source 91 within bias current driver 9 so as to supply a bottom error signal based on the difference between the bottom level Vob and the reference voltage Vbottom to the current source 91 control input. The magnitude of the bias current Ib which bias current driver 8 flows through LD 1 is controlled such that the bottom level Vob of the C/V converter 3 output Vo matches the reference voltage Vbottom, i.e., the bottom error signal from op-amp 52 becomes zero in the level.
In this way, the prior art LD driver provides such a control as continuously maintains constant peak and bottom output optical powers (or a constant extinction ratio) of LD 1 by changing the modulated current Ip and the bias current Ib supplied to LD 1 in response to the photo-current Ipd detected by photodiode 2.
However, in the prior art LD driver, the bottom level or envelope of the output optical power emitted by LD 1 (i.e., the bias current Ib flowed by current source 91 of bias current driver 9) is controlled to be constant on the basis of the reference voltage Vbottom into which the peak level Vop of the C/V converter 3 output Vo is divided. For this reason, it is only after the stabilization of the peak level of the output optical power of LD 1 (or the C/V converter 3 output Vo) that the bottom level of the output optical power (or the bottom reference voltage Vbottom) can be stabilized. Thus, the prior art LD driver has a problem of taking a significant start-up time till the bottom level of output optical power reaches a stable or stationary state.
Also, generally speaking, the efficiency of optical coupling between LD 1 and photodiode 2 can vary in such a wide range such that the maximum coupling efficiency is on the order of ten times the minimum. A lower coupling efficiency causes the photo-current Ipd from photodiode 2 to become smaller, thereby lowering the peak level Vop of the C/V converter 3 output Vo which level is used as the reference for generating the bottom reference voltage Vbottom. Thus, the prior art LD driver still has a problem that if the peak level Vop is small, then the bottom reference voltage Vbottom becomes small, resulting in the bottom level of the output optical power including a larger error.
It is an object of the invention to provide a laser diode driving method and circuit which provides an automatic power control capable of quickly establishing a desired peak level and a desired bottom level of the output optical power of the laser diode with a raised precision.
It is another object of the invention to provide a laser diode system or circuit incorporating such the laser diode driving method and circuit.
It is noted that, in this document, a term xe2x80x9claser diode system or circuitxe2x80x9d means a system or circuit including a laser diode and a driving circuit for the laser diode.
According to the present invention, a laser diode system for emitting a predetermined optical power in response to an input binary signal is provided. The laser diode system comprises: a laser diode, responsive to a driving current applied thereto, for emitting an optical power; a bias current driver for always providing the laser diode with a bias current of a magnitude responsive to a bias control signal; a modulation current driver, responsive to a predetermined level of the input binary signal, for providing a radiation current of a magnitude responsive to a radiation control signal so as to feed the laser diode with the bias current and the radiation current simultaneously; a photodiode, optically coupled with the laser diode, for generating a photo-current proportional to the optical power from the laser diode; a current-to-voltage converter for providing a detected signal or voltage proportional to the optical power from the laser diode; a peak error detector for supplying the bias current driver with the radiation control signal so as to cause a peak level of the detected signal to match a first (or peak) constant reference voltage, the first constant reference voltage being such that a simultaneous application of the bias and radiation currents to the laser diode causes the laser diode to emit the predetermined optical power; a bottom reference voltage generator for generating a second constant reference voltage from the first constant reference voltage; and bottom error detector for supplying the bias means with the bias control signal so as to cause a bottom level of the detected signal to match the second constant reference voltage, the second constant reference voltage being such that the bias current causes the laser diode to emit no laser.
In one embodiment, the bottom reference voltage generator uses a maximum reference voltage instead of the first constant reference voltage. An AGC circuit may be inserted between the current-to-voltage converter and the bottom error detector so as to cause the peak level of a scaled detected signal to match the second constant reference voltage.